A DC-DC converter that generates an output voltage by on/off controlling a switch to boost and step-down a DC input voltage performs feedback control so as to maintain the output voltage supplied to a load to a constant target voltage (for example, U.S. published application No. 2005/0286269 and U.S. published application No. 2007/0120547). As a method to control a switch, a Pulse Width Modulation (PWM) method and a Pulse Frequency Modulation (PFM) method are known.
A PWM method DC-DC converter adjusts an output voltage by fixing a switching frequency and controlling an on-time of a switch by comparing an output of an error amplifier that is generated based on an output voltage and a standard voltage with a saw-tooth signal waveform. The PWM method fixes a switching frequency, thus occurrence of noise due to a switching operation of the known frequency is easily expected, and noise may be easily avoided from the system side during the design.
In the PWM method, an error such as double pulsing may be caused unless frequency characteristics of an error amplifier are set to 1/10 to 1/20 of the switching frequency of a switching element. In other words, if a band of a feedback response according to an output voltage of the DC-DC converter is not sufficiently lower than the switching frequency, an abnormal switching operation may be caused. Thus, there is a drawback in which a high-speed response is not possible for an abrupt change of a load.
The PFM method DC-DC converter adjusts an output voltage by adjusting a frequency of a control signal that controls the on and off of a switch by comparing a fixed output voltage and a standard voltage. Thus, the PFM method DC-DC converter does not need an error amplifier and allows a high-speed response for an abrupt load change.
However, a switching frequency of the above described PFM method DC-DC converter varies due to a difference between an input voltage and an output voltage, and a load change, and thereby noise measures are difficult to apply. As a result, performance of devices that uses a small signal with high frequency such as audio/video devices may be degenerated.